In recent years, a necessity for a high efficiency of a refrigerating apparatus rises more and more. For this necessity, a compressor using a linear motor is expected to reduce the sliding loss drastically because it has a simple mechanical construction. Therefore, the compressor is widely employed to raise the efficiency of the refrigerating apparatus. A conventional linear compressor will be described with reference to the accompanying drawing.
FIG. 21 is a sectional view of the conventional linear compressor. A closed casing (as will be called the “case”) 1 houses a body 3 having a linear motor 2 and holds lubricating oil 4.
The linear motor 2 is constructed of a stator 9 and a mover 12. The stator 9 is composed of a first silicon steel sheet layer (as will be called the “steel sheet layer”) 6 having a hollow cylinder shape, and a second silicon steel sheet layer (as will be called the “steel sheet layer”) 8 of a hollow cylinder shape provided with a coil 7 and formed at a predetermined clearance on the outer circumference of the steel sheet layer 6. Both the steel sheet layers 6 and 8 are held in a frame 5. The mover 12 is loosely inserted between the steel sheet layer 6 and the steel sheet layer 8 and is formed into a hollow cylinder shape by adhering a plurality of magnets 11 to the distal end portion of a magnet shell 10 made of a non-magnetic material. Here, the magnets 11 are generally made of a magnet material of a rare earth element having a ferromagnetic field such as neodymium, and are magnetized in a direction perpendicular to the rocking direction of the mover 12.
A cylinder 14 having a cylindrical bore and a piston 15 inserted reciprocally in the cylinder 14 form a bearing section 16 inbetween. The piston 15 and the magnet shell 10 are integrally formed in a coaxial shape. Moreover, the cylinder 14 is arranged on the inner side of the steel sheet layer 6 formed in a hollow cylinder shape, and has the frame 5 formed on its outer circumference.
The piston 15 is formed into such a hollow cylinder shape as to form a suction passage (as will be called the “passage”) 17 in its internal space. In the open end of the passage 17 on the side of a compression chamber 18, there is mounted a suction valve (as will be called the “valve”) 19. A discharge valve (as will be called the “valve”) 20 is also arranged in the open end of the compression chamber 18.
The cylinder 14, the piston 15 and the steel layers 6 and 8 share their axes. The piston 15 retains the mover 12, through the bearing section 16 between itself and the cylinder 14. As a result, the magnet 11 holds predetermined clearances between itself and the steel sheet layer 6 and the steel sheet layer 8, respectively.
Both an inner resonance spring (as will be called the “spring”) 21 and an outer resonance spring (as will be called the “spring”) 22 are compression coil springs. The spring 21 is arranged to contact with the steel sheet layer 6 and the magnet shell 10, and the spring 22 is arranged to contact with the magnet shell 10 and an outer frame 23. Both the springs 21 and 22 are assembled in compressed states. On the other hand, an oil pump 24 is formed in the bottom portion of the body 3 and is positioned in the lubricating oil 4.
Here will be described the actions of the linear compressor thus constructed.
First of all, when an electric current is fed to magnetize the coil 7, a loop of a series of magnetic fluxes is generated to form a magnetic circuit from the steel sheet layer 6 to the clearance, the magnet 11, the clearance, the steel sheet layer 8, the clearance, the magnet 11, the clearance and the steel sheet layer 6. The magnet 11 is attracted by the magnet poles, which are formed in the steel sheet layer 8 by those magnetic fluxes. When the electric current to the coil 7 then alternates, the mover 12 reciprocates horizontally in FIG. 21 between the steel sheet layer 6 and the steel sheet layer 8. As a result, the piston 15 connected to the mover 12 reciprocates in the cylinder 14. By the reciprocating motion, the coolant gas in the space of the case 1 is sucked via the passage 17 out of the valve 19 into the compression chamber 18 so that it is compressed in the compression chamber 18 and discharged from the valve 20.
The spring 21 is sandwiched between the cylinder 14 and the steel sheet layer 6 and supports the inner side of the mover 12 elastically. The spring 22 supports the outer side of the mover 12 elastically. When the mover 12 reciprocates, the springs 21 and 22 convert the linear reciprocation of the piston 15 and store them as an elastic energy. The spring 21 and the spring 22 induce the resonating motions while converting the stored elastic energy into linear motions.
On the other hand, the oil pump 24 is caused to feed the lubricating oil to the bearing section 16 by the vibrations of the compressor body 3. Such a compressor is disclosed in Japanese Patent Unexamined Publication No. 2001-73942, for example.
In the conventional construction described above, however, the mover 12 rocks between the steel sheet layer 6 and the steel sheet layer 8. Specifically, it is necessary that the mover 12 be prevented from contacting with both the steel sheet layers 6 and 8. For this necessity, the clearances are individually formed between the mover 12 and the steel sheet layers 6 and 8. However, these clearances of the two layers act as magnetic resistances to reduce the magnetic fluxes in proportion to the distances. In order to achieve a thrust needed for driving the mover 12, however, it is necessary to increase the electric current to be fed to the coil 7 to provide an excess of current to compensate for the reduction in the magnetic fluxes due to the two clearances. As a result, the electric current to be inputted must be increased thereby making it difficult to enhance the efficiency.
In order to achieve the thrust needed for driving the mover 12, on the other hand, the magnet 11 has to be enlarged in the conventional linear motor. However, the magnet 11 employs an expensive rare earth element as its material so that the cost drastically rises as the magnet 11 becomes larger.
If there is a difference in the distances between the clearances to be formed between the mover 12 and the steel sheet layers 6 and 8, moreover, an imbalance in the magnetic attractions occurs between the magnet 11 and the steel sheet layers 6 and 8. As a result, a wrenching force perpendicular to the rocking directions of the mover 12 is generated so that a sliding loss occurs at the bearing section 16 composed of the piston 15 and the cylinder 14. Alternatively, an abnormal wear occurs at the bearing section 16 to shorten the lifetime of the compressor. On the other hand, noises are caused by the sliding when the wrenching force between the piston 15 and the cylinder 14 is so high as to cause the wear. Therefore, it is desired that the clearances have an equal distance at any place.
For avoiding this trouble, there is a method for enlarging the distances of the two clearances to thereby reduce the ratio of the differences in the distances. In this construction, however, it is necessary not only to increase the input for the necessary thrust but also to enlarge the magnet 11. It is, therefore, customary to enhance the working precision of the drive system containing the magnet shell 10. In order to enhance the working precision, the magnet shell 10 acting as the moving part has to be thickened for a higher rigidity. As a result, the drive system has an increased weight. And the thrust necessary for driving the mover 12 increases to make it necessary to increase the electric current to be fed to the coil 7. Moreover, the load to be borne by the bearing section 16 rises to increase the sliding loss.
On the other hand, the magnet shell 10 and the piston 15 are connected to each other outside of the steel sheet layers 6 and 8, and the spring 21 is arranged between the magnet shell 10 and the steel sheet layer 6. Therefore, the magnet shell 10 has an axially long shape. In this shape, the rigidity is liable to become low especially at the distal end portion carrying the magnet 11. For retaining the precision, therefore, it is necessary to enhance the rigidity. For this necessity, countermeasures are taken by making the sheet thicker to thereby increase the weight more.
Moreover, it is essential for reducing the imbalance of the magnetic attractions that the assembly be made highly precise for even clearances, in addition to the working precision. Because of the two clearances, both the clearances inside and outside of the magnet shell 10 have to be managed thereby requiring strict management of the precision during manufacturing thereby raising the cost.
If the magnet shell 10 of the hollow cylinder shape is formed of a thin sheet for the lower weight, the rigidity of the magnet shell 10 or its supporting structure is insufficient. As a consequence, the imbalance of the magnetic attractions occurs due to the variation in the parts precision, the assembly precision or the magnetic force of the magnet 11, and the supporting structure is deformed so that the magnet 11 is radially attracted. Then, the magnet 11 and the steel sheet layers 6 and 8 approach respectively in the two clearances of the two layers to thereby cause the vicious circle, in which the magnetic attractions are intensified more to make the eccentricity of the magnet 11 more. As a result, the magnet shell 10 is subjected to a serious force so that it is deformed to generate noises. In the worst case, the steel sheet layers 6 and 8 and the magnet 11 collide against each other to cause breakage.